The present invention relates generally to new manufacturing processes for machining (i.e. removing material) at the nano-scale. The invention specifically relates to methods and devices for machining a workpiece at the nanoscale level using carbon nanotubes as the machining tool.
Conventional machining and micro/nano fabrication processes transfer energy into small spatial regions to remove material from a workpiece. For instance, traditional cutting with a lathe transfers energy from a tool to a workpiece through the cutting edge of the tool and focused ion beam machining transfers energy to a workpiece with a narrow beam of ions that strike the workpiece""s surface. To achieve nanoscale machining, which is the removal of material near the atomic scale (1 nm-100 nm), energy must be concentrated spatially with nearly atomic resolution and also sufficient to break bonds in crystal lattices. If the energy is too excessive, then nearby atoms will also be effected and nanoscale resolution cannot be attained. Therefore, to achieve nanoscale machining, energy should be within specific limits and spatially controlled to attain patterns.
Current micromachining processes can be categorized as either parallel techniques, which simultaneously remove material from a workpiece at many locations, or serial techniques, which sequentially remove material from a single location. Common parallel techniques include surface micromachining, bulk micromachining, and laser micromachining. Common serial techniques include mechanical cutting, micro electro discharge machining (EDM), electron beam machining, ion beam machining, and laser beam machining. Parallel techniques are well suited to economically fabricating parts with 2-D geometry in batches. Serial techniques are more suitable for complex 3-D micromachining, but cannot be used for fabricating batches of parts.
Both parallel and serial micromachining techniques require a shape-generation mechanism. In parallel techniques, the shape-generating mechanism is usually a mask used during lithography. In e-beam, ion-beam, and laser micromachining, the shape-generating mechanism is the column and tip of the energy beam. In micro EDM, the electrode is the shape-generating mechanism. The physical processes that produce the shape-generating mechanism limit the resolution and size of the geometry that can be machined. Most micromachining processes are limited to feature sizes and resolution exceeding 1 xcexcm. For instance, electrodes for micro EDM are commonly manufactured using wire electro discharge grinding (WEDG). WEDG can produce cylindrical electrodes with diameters down to about 5 xcexcm. Consequently, the minimum feature-size producible with micro EDM is also on the order of 5 xcexcm.
Because of the limits in current micro- and nano-machining processes, there is a need in the art for an entirely new approach for nano-machining desired workpieces. The present invention relates to a nanotool with nanostructures less than 100 nm in size, and to methods of using the nanotool to remove material from a workpiece in desired two-dimensional (2D) and three-dimensional (3D) patterns. Nanotubes, which are carbon structures with diameters varying from 2 nm to 100 nm and lengths up to several microns, are used for shape generation and machining due to their unique physical properties for nano-machining via electrical discharges or electron emission. The present invention employs electron beams emitted from carbon nanotubes as a source of energy with nanoscale resolution and manufactures patterns by either using a predetermined pattern of nanotubes, or by relative motion between the workpiece and nanotubes.
The natural size of nanotubes and the inherent precision in their fabrication processes enable the manufacture of nano-scale tools for both serial and parallel nano-machining. For serial nano-machining, a single nanotube may be grown on an electrode. For parallel nano-machining, a 2D pattern of nanotubes may be grown on a conductive electrode substrate. The methods and devices of the present invention enable nano-machining by improving resolution and reducing minimum machined feature sizes by two to three orders of magnitude in comparison to conventional micro- and nano-machining technologies.
In accordance with the purposes of the present invention as described herein, in one aspect the present invention provides a method for machining a nanometer-scale pattern on a surface of an electrically conductive workpiece, comprising the steps of placing a nanotool in substantial proximity to the conductive workpiece surface, creating an electrical potential difference between the nanotool and the workpiece surface to cause an electron beam to emit from the nanotool and strike the conductive workpiece surface, resulting in evaporation of nanoscale quantities of material from the workpiece surface, and applying a vacuum to remove evaporated material from the workpiece surface. The electric field potential established to cause the electron beam to emit from the nanotool will typically be at least 1 V/xcexcm. The nanotool may comprise at least one nanotube supported on an electrically conductive base. The method of the present invention may include the further step of exciting the workpiece to a threshold energy prior to contacting the workpiece with the electron beam to evaporate material therefrom.
In one embodiment of the method of the present invention, the workpiece or the nanotool may be moved relative to one another to remove material from the workpiece in accordance with a predetermined pattern. The nanotool may comprise a single nanotube supported by an electrically conductive base, or may comprise a plurality of substantially aligned nanotubes supported on the base. The plurality of nanotubes may be confined to one or more patterned regions of the electrically conductive base. It will be appreciated that for some applications this feature obviates the need to move either workpiece or nanotool relative to one another to machine the corresponding pattern into the workpiece.
Nanotubes suitable for the present invention include carbon nanotubes. However, it will be appreciated that any substance capable of forming a nanotube for emitting an electron beam in response to an electrical field may be used, such as tungsten, nickel, and the like. The nanotubes of the present invention may be single-walled or multi-walled nanotubes, and typically have a diameter of from about 1 to about 100 nanometers.
The nanotool conductive base may be fabricated from any suitably electrically conductive metal or polymer, including but not limited to materials selected from the group consisting of silicon nitride, titanium nitride, tungsten carbide, tantalum nitride, porous silicon, nickel, cobalt, gold, aluminum, polycrystalline diamond, and any combination thereof.
The conductive workpiece may be fabricated from any suitably conductive metal or polymer, and may be selected from materials including, but not limited to, the group consisting of aluminum, copper, silver, gold, polymethylmethacrylate, and any combination thereof. The workpiece may be deposited as a thin film on a substrate, with the thin film having a depth of up to 5 microns. The workpiece substrate may be fabricated from any material which is substantially transparent to a laser beam. It will be appreciated that this feature allows use of a laser to heat the workpiece from a first surface, while the nanotool of the invention is used to remove material from the obverse surface of the workpiece. The substrate may be fabricated from materials selected from the group consisting of, but not limited to, single-crystal quartz, amorphous quartz, silicon, and any combination thereof.
Excitation of the workpiece to a threshold energy may be achieved by heating. The workpiece may be heated by localized heating, by radiative heating, by conductive heating, by resistive heating, or any combination thereof. In one embodiment of the method of the present invention, localized heating may be provided by targeting a laser beam to an area substantially corresponding to the pattern to be machined. Targeting of the laser beam may be accomplished by narrowly restricting the beam width, or by masking the workpiece such that only an area corresponding to the desired pattern to be machined is exposed to the laser beam. Such masking substances and methods are known in the art. Any suitable continuous wave laser providing a light beam having a wavelength of from about 0.3 xcexcm to about 0.7 xcexcm may be used for localized heating of the workpiece. Examples include, but should not be limited to, argon lasers, HeCd lasers, or HeNe lasers.
To generate the nanotool electron beam for evaporation of material from the workpiece, an electrical potential of from about 500 V to about 5 kV may be applied to the nanotool. A vacuum of up to 10xe2x88x925 torr may be applied to the nanotool and the workpiece to remove evaporated material. This step allows withdrawal of material removed from the surface of the workpiece by the nanotool using the applied vacuum pressure. Accordingly, no additional removal means (such as an inert sweep gas) is required.
In another aspect of the present invention, a device for machining a nanometer-scale pattern on a surface of an electrically conductive workpiece is provided, comprising a vessel having a top, a bottom, and at least one side wall defining an interior chamber to which a vacuum of up to 10xe2x88x925 torr may be applied. In the interior chamber, an apparatus is placed, the apparatus comprising a first support for holding a conductive workpiece, and a second support for holding a nanotool as described above in relative proximity to the conductive workpieces. An electrical source is provided for applying an electrical current to the nanotool. The electrical source of the present device may be any electrical source capable of creating an electrical potential difference of at least 1 V/xcexcm between the nanotool and the workpiece. As described above, applying this electrical current creates an electrical potential difference between the nanotool and the workpiece to cause an electron beam to emit from the nanotool and strike the conductive workpiece surface, evaporating material therefrom. The device of the present invention may further include a heater for heating at least a portion of the workpiece surface by localized heating, by radiative heating, by conductive heating, by resistive heating, or by any combination thereof.
The first support may be a leveling support for holding the conductive workpiece. The second support may be a nanopositioning stage for translation of the nanotool in nanometer increments. It will be appreciated that these features allow precise movement of the workpiece and nanotool relative to one another.
As described above, the localized heater may be a laser emitting a light beam having a wavelength of from about 0.3 xcexcm to about 0.7 xcexcm, and targeted to heat at least a portion of the workpiece surface. In cases when it is desirable to position the laser externally of the vessel of the present device, the vessel may include at least one port fabricated from a material substantially transparent to a laser beam, allowing the workpiece to be targeted by the laser without introducing the laser to the interior of the vessel. In such cases, the vessel may include at least one mirror surface in the interior chamber to redirect the laser beam to contact and heat at least a portion of the workpiece surface. As noted above, a masking grid fabricated of materials known in the art may be interposed between the point of origin of the laser beam and the workpiece surface, thereby partially preventing contact of the laser beam with the workpiece such that only the unmasked portion of the workpiece is heated. Of course, other means for heating the workpiece may be included, such as a heating element in a spaced orientation to the conductive workpiece (radiative heating) or in thermal contact with the workpiece (conductive heating), or an electrical current passed through the workpiece (resistive heating).
In still yet another aspect of the present invention, a nanotool for removing nanoscale quantities of material from a surface of an electrically conductive workpiece is provided, comprising at least one nanotube supported on an electrically conductive base and capable of emitting an electron beam in response to an electrical field applied to the conductive base. Thus, the nanotool will include at least one contact for receiving an electrical signal.
The nanotool may comprise a single nanotube supported on the base, or may comprise a plurality of substantially aligned nanotubes supported on the base. As noted above, the nanotool may comprise a plurality of substantially aligned nanotubes confined to one or more patterned regions of the conductive base. As also described above, any suitable nanotube may be used, such as a carbon nanotube, or a nanotube synthesized from tungsten, nickel, and the like. Both single- and multi-walled nanotubes may be utilized in fabricating the nanotool of the present invention. Typically, the nanotubes will have a diameter of from about 1 to about 100 nanometers. The conductive base may be fabricated from any suitably conductive metal or polymer as described above.
Other objects and applications of the present invention will become apparent to those skilled in this art from the following description wherein there is shown and described a preferred embodiment of this invention, simply by way of illustration of the modes currently best suited to carry out the invention. As it will be realized, the invention is capable of other different embodiments and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.